1. Field of the Invention
The present invention relates to an electroacoustic transducer that generates a sound by means of electromagnetic acoustic conversion.
2. Description of the Related Art
FIG. 10 is a perspective view showing an example of an electroacoustic transducer of the conventional art. The electroacoustic transducer 90 is configured by: a diaphragm 93 which is made of a magnetic material, and in which a magnetic piece 94 is fixed to the center; an electromagnetic coil (not shown) which supplies an oscillating magnetic field to the diaphragm 93; a housing member 91 which accommodates the diaphragm 93 and the electromagnetic coil; etc. Since a sound release opening 91b the diameter of which is larger than the magnetic piece 94 is formed in a top plate 91a which is opposed to the diaphragm 93, such a transducer is usually called an open-type electroacoustic transducer.
When an electric oscillating signal is supplied to the electromagnetic coil, the diaphragm 93 is oscillated by an oscillating magnetic field generated by the electromagnetic coil, to generate a sound. The sound is released to the outside through the sound release opening 91b. 
A support ring which supports a peripheral portion of the diaphragm 93, and a ring-like magnet which supplies a static magnetic field are accommodated in the electroacoustic transducer 90. When the dimensions of the magnet are designed so as to be as large as possible in order to enhance the magnetic field generated by the magnet, the magnet is structured to be in close contact with the inner side of the support ring.
On the other hand, when the back space of the diaphragm is small in volume, the resonance frequency f0 of the diaphragm is raised by the air damping effect. Therefore, the air damping effect exerts a larger effect as the transducer is smaller in size.
Under those circumstances, when a magnet material having a high maximum energy product (BHmax) is selected as the magnet material so as to reduce the size of the magnet, a space can be ensured between the magnet and the support ring. Therefore, the volume of the back space can be increased. In this case, the magnet is separated from the support ring, and hence a fixation method using an adhesive agent may be employed. However, such a fixation method has fears such as that the adhesive agent deteriorates with age, and that the production cost is increased by conducting a bonding step.
The present invention has been made to solve the above problems, and therefore an object of the invention is to provide an electroacoustic transducer in which a magnet can be surely fixed while ensuring a large back space of a diaphragm, and which can be miniaturized and produce a high sound pressure.
According to a first aspect of the invention, there is provided an electroacoustic transducer comprising: a base member made of a magnetic material; a magnetic core made of a magnetic material and upstanding on said base member; a diaphragm made of a magnetic material and spaced from a leading end of said magnetic core; a magnetic field generating member which cooperates with said base member, said magnetic core and said diaphragm to constitute a magnetic circuit, for supplying a static magnetic field; and a coil placed around said magnetic core for supplying an oscillating magnetic field to said magnetic circuit; wherein said magnetic field generating member comprises multiple ring members whose faces opposing said diaphragm have respectively magnetic poles of the same polarity, respectively.
In the first aspect of the invention, the magnetic field generating member comprises multiple ring members whose faces opposing the diaphragm have respectively magnetic poles of the same polarity, respectively. Therefore, a magnetic field loop which starts from the diaphragm opposing faces of the ring members toward the center of the diaphragm and then returns to the bottom faces of the ring members through the magnetic core and the base member can be multiplexed. As compared with the case of a single magnetic field loop, consequently, the magnetic attractive force for the diaphragm can be remarkably improved. As a result, the pressure level of a sound generated by the diaphragm can be raised, and the frequency characteristics of a sound can be flattened.
According to a second aspect of the invention, an outermost one of the ring members supports a peripheral portion of the diaphragm.
Therefore, when a peripheral portion of the diaphragm is supported by the outermost ring member, the ring members can function as both of a magnet and a support ring. Therefore, the space reduction is attained, so that the transducer can be miniaturized.
According to a third aspect of the invention, the ring members are integrated together with coupling members which are intermittently arranged in a circumferential direction.
Therefore, when the ring members are coupled with one another by coupling members which are intermittently arranged, spaces can be formed between the coupling members, and hence the back space of the diaphragm can be increased. As a result, even when the transducer is miniaturized, influences due to the air damping effect can be reduced.
Since the ring members are integrated together, the production cost can be lowered by reduction of the numbers of components and fixing positions, and the positioning accuracy of the components can be improved.
According to a fourth aspect of the invention, communication grooves through which an inner side and an outer side communicate with each other are formed in a bottom face of an outermost one of the ring members.
Therefore, when the communication grooves are formed in the bottom face of the outermost ring member, routes through which the back space of the diaphragm communicates with the external space can be ensured. As a result, the hermeticity of the back space is lowered, thereby being capable of further reducing an influence of the air damping effect.
According to a fifth aspect of the invention, there is further provided a magnetic piece fixed to the diaphragm and having a diameter which is equal to or larger than an inner diameter of an innermost one of the ring members.
Therefore, since the diameter of the magnetic piece is equal to or larger than the inner diameter of the innermost ring member, lines of magnetic force passing through the magnetic piece are increased in number, and hence the magnetic attractive force for the diaphragm can be enhanced.
According to a sixth aspect of the invention, there is provided an electroacoustic transducer comprising: a base member made of a magnetic material; a magnetic core made of a magnetic material and upstanding on said base member; a diaphragm made of a magnetic material and spaced from a leading end of said magnetic core; a magnetic field generating member which cooperates with said base member, said magnetic core and said diaphragm to constitute a magnetic circuit, for supplying a static magnetic field; and a coil placed around said magnetic core, for supplying an oscillating magnetic field to said magnetic circuit; wherein said magnetic field generating member comprises a ring member and a plurality of rib members that protrude inward.
According to the invention, the magnetic field generating member is configured by the ring member, and the plurality of rib members which inward protrude. Therefore, spaces can be formed between the rib members while ensuring the volume of the magnet, and hence the back space of the diaphragm can be increased. As a result, even when the transducer is miniaturized, influences due to the air damping effect can be reduced.
According to a seventh aspect of the invention, the ring member supports a peripheral portion of the diaphragm.
Therefore, when a peripheral portion of the diaphragm is supported by the ring member, the ring member can function as both of a magnet and a support ring. Therefore, the space reduction is attained, thereby being capable of miniaturizing the transducer.
According to an eighth aspect of the invention, communication grooves through which an inner side and an outer side communicate with each other are formed in a bottom face of the ring member.
Therefore, when the communication grooves are formed in the bottom space of the ring member, routes through which the back face of the diaphragm communicates with the external space can be ensured. Therefore, the hermeticity of the back space is lowered, thereby being capable of further reducing an influence of the air damping effect.
According to an eighth aspect of the invention, there is further provided a magnetic piece fixed to the diaphragm and having a diameter which is equal to or larger than an innermost diameter of the rib members.
Therefore, since the diameter of the magnetic piece is equal to or larger than the inner diameter of the innermost rib member, lines of magnetic force passing through the magnetic piece are increased in number, and hence the magnetic attractive force for the diaphragm can be enhanced.